To date, composite preparations comprising plural materials have been developed in order to attain unique characteristics that are not found in any single material. As an example, glass fiber reinforced plastic has been widely used. The successful development of carbon fibers and reinforced plastic containing carbon fibers (CFRP) has brought such composite materials into general use.
These materials have been widely used in sporting goods and so on, and have also gained much attention as a light weight-, high intensity- and high elastic modulus-structural material for aircrafts. In addition to the fiber reinforced materials mentioned above, composite materials reinforced with minute particles have also been successfully developed. Composite materials, while generally regarded as structural materials for their structural properties, such as strength and heat resistance, are increasingly being recognized as functional materials for their electric, electronic, optical, and chemical characteristics.
As various electronic devices increases, problems such as malfunction of devices caused by static electricity and electromagnetic wave interference caused by noises from certain electronic components affect peripheral equipments are also on the rise. In order to solve these problems, materials that have excellent functional characteristics such as conductivities and damping abilities are required in this field. Traditional conductive polymer materials currently in use are made by blending high conductive fillers with low conductive polymers. In such materials, metallic fibers, metallic powder, carbon black, carbon fibers, and other similar materials are generally used as conductive fillers. However, there are several drawbacks in these types of materials. For example, when using metallic fibers and metallic powders as the conductive filler, the materials thus obtained have poor corrosion resistance and mechanical strength. When using carbon fibers as the conductive filler, although a predetermined strength and elastic modulus may be obtained by adding relatively large amounts of the filler, electrical conductivity generally cannot be greatly enhanced by this approach. If one attempts to attain a predetermined conductivity by adding a large amount of filler, one would invariably degrade the intrinsic properties of the original resin material. Incidentally, with respect to a carbon fiber, it is expected that its conductivity imparting effect increases as its diameter becomes smaller at an equivalent additive amount, because the contact area between the fiber and the matrix resin increases.
Carbon fibers may be manufactured by subjecting a precursor organic polymer, particularly, a continuous filament of cellulose or polyacrylonitrile, to thermal decomposition under a well controlled condition, in which a forced tension on the precursor polymer is carefully maintained in order to achieve a good orientation of anisotropic sheets of carbon in the final product. In such manufacturing processes, the level of material loss during carbonization is high and the carbonization rate is slow. Therefore, carbon fibers made by these processes tend to be expensive.
In recent years, a different class of carbon fibers, i.e., fine carbon fibers such as carbon nano structures, exemplified by the carbon nanotubes (hereinafter, referred to also as “CNT”), have been attracting public attention.
The graphite layers that make up the carbon nano structures are materials normally comprised of regular arrays of six-membered ring whose structures can bring about specific electrical properties, as well as chemically, mechanically, and thermal stable properties. As long as such fine carbon fibers can retain such properties upon combining and dispersion into solid materials, including various resins, ceramics, metals, etc., or into liquid materials, including fuels, lubricant agents, etc., their usefulness as additives for improving material properties can be expected.
On the other hand, however, such fine carbon fibers unfortunately show an aggregate state even just after their synthesis. When these aggregates are used as-is, the fine carbon fibers would be poorly dispersed into the matrix, and thus the product obtained would not benefit from the desired properties of the nano structures. Accordingly, given a desired property such as electrical conductivity for a matrix such as a resin, it is necessary that the fine carbon fibers would be added in a large amount.
Japanese patent No. 2862578 discloses a resin composition comprising aggregates, wherein each of the aggregates is composed of mutually entangled carbon fibrils having 3.5–70 nm in diameter, and wherein the aggregates possess a diameter in the range of 0.10 to 0.25 mm with a maximum diameter of not more than 0.25 mm. It is noted that the numeric data such as the maximum diameter, diameter, etc., for the carbon fibril aggregates are those measured prior to combining with resin, as is clear from the description in the examples and other parts of the patent literature. The related parts of Japanese patent No. 2862578 are incorporated herein by reference.
JP-2004-119386A discloses a composite material, wherein a carbon fibrous material is added to the matrix. The carbon fibrous material is mainly comprised of aggregates, each of which is composed of carbon fibers having 50–5000 nm in diameter. The mutual contacting points among the carbon fibers are fixed with carbonized carbonaceous substance. Each of the aggregates has a size of 5 μm–500 μm. In this reference, the numeric data such as the size of aggregates, etc., are those measured prior to combining with resin. The related parts of JP-2004-119386A are incorporated herein by reference.
Using carbon fiber aggregates such as those described above, it is expected that the dispersibility of carbon nano structures within a resin matrix will improve to a certain degree as compared with that of using bigger lumps of carbon fibers. Aggregates prepared by dispersing carbon fibrils under a certain shearing force, such as in a vibrating ball mill or the like, according to Japanese patent No. 2862578, however, have relative high bulk densities. Thus, they do not fulfill the need for ideal additives that are capable of improving various characteristics of a matrix, such as electrical conductivity, at small dosages.
JP-2004-119386A discloses a carbon fibrous structure, which is manufactured by heating carbon fibers in a state such that mutual contacting points among the carbon fibers are formed by compression molding after synthesis of the carbon fibers, and wherein the fixing of the fibers at the contacting points is done by carbonization of organic residues primarily attached to the surface of the carbon fibers, or carbonization of an organic compound additionally added as a binder. Since the fixing of carbon fibers is performed by such a heat treatment after synthesis of the carbon fibers, the affixing forces at the contacting points are weak and do not result in good electrical properties of the carbon fibrous structures. When these carbon fibrous structures are added to a matrix such as a resin, the carbon fibers fixed at the contacting points are easily detached from each other, and the carbon fibrous structures are no longer maintained in the matrix. Therefore, it is not possible to construct preferable conductive paths in a matrix such that good electrical properties may be conferred on the matrix by a small additive amount of the carbon fibrous structures. Furthermore, when a binder is added to promote fixing and carbonization at the contacting points, fibers in the obtained fibrous structures would have large diameters and inferior surface characteristics because the added binder is attached to the whole surface areas of the fibers rather than to limited areas on the contacting points.